New drugs aim to disarm the immune system’s ‘atomic bomb’ cells
Seeking a new treatment for people who have dangerous blockages in their coronary arteries, doctors in London are trying to disarm the body’s own defenders. The 90 patients in the study receive the usual treatments for heart disease, such as small tubes called stents to prop open their narrowed arteries. But half of the patients in the phase II trial, conducted by clinical pharmacologist Albert Ferro of King’s College London and colleagues, also pop pills targeting a class of immune cells called neutrophils. Researchers think that by invading the fatty obstructions, or plaques, in clogged arteries, neutrophils make them even more dangerous. The drug is designed to steer the cells away.
Interfering with neutrophils is an audacious idea. Neutrophils make up some 70% of the white cells in blood, with billions spawned every day by stem cells nestled in the bone marrow. The cells patrol the bloodstream and wage war on pathogens, and their defensive role is so vital that people who lose their neutrophils in the course of cancer treatment can die from infections.
But their scorched-earth tactics are tough on the body. “They are the classic double-edged sword,” says immunologist Michael Fessler of the U.S. National Institute of Environmental Health Sciences. For example, the enzyme neutrophil elastase, which neutrophils release to kill invading bacteria, also erodes the extracellular matrix, the mesh of proteins and sugars that cradles cells and provides structural support for tissue.
Neutrophils can also malfunction. In some conditions, such as chronic obstructive pulmonary disease (COPD), they seem to lose their sense of direction: Instead of homing in on bacteria, they wander. Researchers suspect these vagrant neutrophils release neutrophil elastase and other molecules at the wrong spots, including the lungs, causing the tissue damage seen in COPD. And as people age, neutrophils become less effective at combating infections and more harmful to the body.
“We are now beginning to understand that neutrophils are linked to many diseases that blight our population,” says pulmonologist Elizabeth Sapey of the University of Birmingham in the United Kingdom. With that realization has come optimism that these cells aren’t off-limits for medical interventions. Researchers have been cautious about targeting neutrophils, fearing that doing so would leave patients at the mercy of pathogens. “The age-old idea was that you couldn’t touch neutrophils,” Sapey says. But a series of clinical trials has offered reassurance that it can be safe to restrain the cells.
Although some of those trials yielded mediocre results, researchers have pressed on—and their persistence may be paying off. Two clinical trials with neutrophil-focused drug candidates reported encouraging findings last year; one could lead to the approval of the first medication targeting these immune cells, in a rare autoimmune condition. The trial Ferro and his colleagues are conducting, the first to attempt to stem cardiovascular disease by interfering with neutrophils, is expected to have results by 2021 or 2022. And a burst of new findings about the cells’ diversity and behavior is likely to inspire new ways to manipulate them. “There is real hope in this field,” Sapey says.
NEUTROPHILS FLOCK to the site of an injury or infection, wherever it occurs. The cells navigate to trouble spots with the help of receptors on their surface, such as one known as CXCR2, which detects a trail of alarm molecules released by damaged tissue. If the cells happen upon microbes, they launch a multipronged attack, gobbling the pathogens, spilling corrosive chemicals, and sometimes launching webs of DNA, known as neutrophil extracellular traps (NETs), that snag and kill the invaders.
Hematopoietic stem cell transplants given to some cancer patients to replace their bone marrow underscore the importance of these defenses. “If the graft doesn’t take, you will have no neutrophils, and that is not survivable,” says immunologist Klaus Ley of the La Jolla Institute for Immunology.
Yet studies over the past 2 decades have darkened their image. The cells in effect “drop atomic bombs” in the body, says immunologist Paul Kubes of the University of Calgary. Neutrophil elastase and other chemicals they discharge may drive inflammation and damage to the airways in cystic fibrosis and intractable cases of asthma. NETs, for their part, can provoke the immune system to attack a patient’s own cells in the autoimmune disease lupus, and they spur formation of blood clots in the potentially lethal condition deep-vein thrombosis. NETs can also worsen arterial plaques by promoting inflammation, cell biologist Venizelos Papayannopoulos of the Francis Crick Institute in London and colleagues found in 2015. Neutrophils can even abet cancer, spurring growth of new blood vessels that feed tumors and helping the abnormal cells spread to other parts of the body.
More than 10 years ago, such disease links prompted some companies to begin to develop compounds that inhibit neutrophil elastase. Others tried to restore neutrophils’ navigational abilities, which are faulty in COPD and other conditions, by blocking the protein PI3K, an enzyme involved in controlling cell movement.
Neutrophils are linked to many diseases that blight our population.
Elizabeth Sapey, University of Birmingham
More than a dozen clinical trials scrutinized these potential drugs in many conditions. The good news is that the compounds didn’t cripple defenses against infections. But most of the trials found minimal benefits. As a result, several big pharmaceutical companies have given up. In 2019, GlaxoSmithKline jettisoned danirixin and nemiralisib, the two neutrophil-targeting drug candidates it had been developing for lung diseases. Merck and AstraZeneca have also recently abandoned once-promising compounds.
But researchers say there’s still hope, arguing they haven’t yet pinned down what doses to use, how best to deliver potential drugs, and which aspects of neutrophil biology to target. In trials for COPD and other lung disorders, Sapey notes, patients inhaled the compounds. But because most neutrophils ply the bloodstream, inhaled drugs may not reach faulty cells, she says. Nervous about side effects, investigators may also have kept doses too low.
Hematologist and oncologist Steven Pavletic of the U.S. National Cancer Institute and his colleagues think they can do better. In a phase I safety trial, Pavletic and his team have started to give larger amounts of the neutrophil elastase inhibitor alvelestat to patients with the lung condition bronchiolitis obliterans. The ailment strikes a significant percentage of people who receive hematopoietic stem cell transplants during cancer treatment, developing when immune cells from the transplant attack the recipient’s own tissue. Neutrophils swarm into the lungs and may encourage scar tissue to build up in the bronchioles, the lungs’ smallest airways, which can cause them to clog. For transplant recipients, bronchiolitis obliterans is like having an unstoppable asthma attack, Pavletic says.
He adds that alvelestat probably won’t reverse bronchiolitis obliterans, but by blocking neutrophil elastase, it could stop the disease from getting worse. “Even a marginal improvement could lead to a major improvement in lung function.”
Researchers are further along in evaluating a way to target neutrophils to treat a rare autoimmune disease in which the immune system produces antibodies against its own neutrophils—a condition known as antineutrophil cytoplasmic autoantibody (ANCA) vasculitis. The antibodies glom onto the cells, which then get stuck in small blood vessels and release their chemical load, leading to inflammation that can cause kidney damage and other problems. In the first year after diagnosis, patients with the disease are nine times more likely to die than people in the general population, and current treatments, which include steroids and immune-suppressing drugs, are one reason for this high mortality, notes nephrologist David Jayne of the University of Cambridge.
Released by neutrophils, vesicles (yellow) carrying protein-degrading enzymes can bind to and damage collagen fibrils (blue) in connective tissue.
In a recent trial involving 331 ANCA vasculitis patients, Jayne and colleagues tested the experimental drug avacopan, which stymies C5a, a protein in the blood that helps spur neutrophils to release their inflammation-promoting contents. The results of the phase III trial, announced late last year, revealed that the drug was about 20% more likely to produce remissions after 1 year than steroids were. It improved patients’ kidney function and was also less harmful than steroids. Jayne predicts avacopan will be approved by the U.S. Food and Drug Administration and “will be a complete replacement for steroids” in treatment of ANCA vasculitis.
Although many efforts to target neutrophils aim to inhibit them, Sapey and her colleagues believe rejuvenating neutrophils could help boost immune function in older people. An existing drug, the cholesterol-lowering medication simvastatin, may do the job, they found. Simvastatin had caught their attention because some evidence suggested people taking it for high cholesterol are less vulnerable to infections. When the team members exposed neutrophils to simvastatin in the lab, they found it boosts the cells’ migration accuracy and improves their performance in other ways. And in a phase II trial of the drug in 62 older people with bacterial pneumonia and sepsis, Sapey and her colleagues determined that adding simvastatin to the normal treatment regimen reduced the severity of infections and allowed the patients to spend more time out of the hospital over the next year.
“That was highly unexpected and very exciting,” says Sapey, whose team reported the findings last year in the American Journal of Respiratory and Critical Care Medicine. They are now planning a larger study of the drug, and she says it might also work against other common bacterial problems in the elderly, such as urinary tract and skin infections.
THE THERAPEUTIC POSSIBILITIES are likely to broaden as researchers learn more about the biology of neutrophils. The cells have traditionally been seen as the immune system’s cannon fodder—simple, expendable killers. But neutrophils, it turns out, aren’t so simple after all. “They are not merely the cells that clear microbes from infected sites,” Papayannopoulos says.
“The most exciting thing is that the cells are much more heterogeneous than we thought,” says immunologist Leo Koenderman of University Medical Center Utrecht in the Netherlands. With techniques such as single-cell RNA sequencing, researchers have discovered that neutrophils come in multiple varieties. And their properties can vary by time of day. In humans, the cells are much more aggressive toward pathogens at night than during daylight hours, and they crank up different genes depending on the hour.
This diversity enables the cells to specialize and take on more tasks than researchers expected. Some neutrophils have moved up the chain of command and help control the activity of other immune cells. For example, they can either spur T cells to attack pathogens or rein them in. The cells can also take on alternative roles when they enter damaged tissue. If they encounter microbes, they become fighters. But if an injury is not infected, they become healers. Chemicals they release can stimulate new blood vessels to form and spark the production of replacement cells.
Intravital microscopy, which reveals the movements of cells within the body, suggests the lives of these immune cells are eventful. Researchers long assumed that once neutrophils left the bloodstream and moved into infected or injured tissues, they would eventually self-destruct, allowing the inflammation at the site to clear. But several microscopy studies have revealed more complex migrations. In a 2011 study, for example, Anna Huttenlocher of the University of Wisconsin, Madison, and colleagues tracked neutrophils in zebrafish that had wounds on their fins. The scientists found that individual neutrophils moved into and out of a wound several times before eventually departing for good.
Now, Ferro and his colleagues are trying to interrupt one leg of the neutrophils’ journey. In patients with heart disease, the cells worm into the fatty plaques lining the coronary arteries. There, they release chemicals, including neutrophil elastase, that may make these plaques more likely to fracture and spawn blood clots, which can trigger heart attacks.
The drug candidate that Ferro and colleagues are testing in people with serious plaques, AZD5069, blocks CXCR2, the receptor that helps neutrophils navigate to infected and inflamed locations. In animal tests, the compound deters neutrophils from entering sites of inflammation.
Death rates from cardiovascular diseases have been dropping for decades in many countries thanks to lifestyle changes, such as a decline in smoking, as well as drugs that lower cholesterol and combat blood clotting. But heart disease still kills more than 600,000 people in the United States every year. “There’s clearly a big gap that remains in therapy for these patients,” Ferro says. Within a couple of years, he hopes to know whether deflecting neutrophils can help.
Some researchers remain skeptical that the current crop of experimental neutrophil-targeting drugs will work outside certain rare diseases. The cells have multiple redundant pathways that control their activity, Koenderman notes. A neutrophil “is extremely robust,” he says. The idea that a single drug could correct its misbehavior “is pretty naïve,” he says.
However, researchers are already developing new approaches, including DNA-destroying enzymes that might slice up NETs and prevent blood clots. And as scientists dig deeper into the cells’ biology, they may find new ways to keep these immune soldiers in check, Fessler says. “There is hope that with increasing understanding of neutrophils, we will have more sophisticated approaches down the line.”